There has been much work for providing frequency conversion of the output from presently available laser diode sources to produce wavelengths not readily available from these sources. The most attractive alternative for frequency conversion, such as frequency doubling, sum frequency generation and difference frequency generation, is quasi-phase matching (QPM) of an input radiation beam or beams from a laser diode source and their harmonic waves in second order optical crystals, such as inorganic crystals LiNbO.sub.3, LiTaO.sub.3 and KTP as well as in organic materials, such as polymeric mediums. In the case of such crystals, the QPM conditions must be satisfied between the interacting waves in order to achieve efficient nonlinear optical interaction. QPM allows interactions between lightwaves or radiation polarized such that the nonlinearity is maximized and allows interactions to occur in the crystal for which birefringent phase matching is not possible. Compared to birefringent phase matching, QPM allows both access to new wavelengths and higher conversion efficiencies. Since the refractive index of the crystal is dependent upon wavelength of the light to be converted, it is necessary to provide a periodic inverted domain structure within the crystal so as to have domains in the crystal of nonlinear optical coefficient of periodic inverted sign, e.g., two or more regions or domains of different states of ferroelectric polarization transverse to the path of light to be converted. First order QPM requires sign reversals of the effective nonlinear coefficient with a period equal to two coherence lengths. The light waves produced by the nonlinear polarization periodic pattern in the crystal are in phase at the given wavelength so that the waves intensify each other. In particular, QPM allows nonlinear interactions between waves polarized on the z axis for which the maximum nonlinear coefficient or tensor, d.sub.33, is utilized.
To date, the frequency conversion that is highly desirable is that which generates visible light in the "blue" radiation spectrum, such as wavelengths in the range of about 390 nm to 492 nm, which has many applications such is in color display devices, color projectors and color printers.
In practice, the ability to create finely spaced domains with sufficiently accurate periodicity and well defined domain walls in the crystal is a challenging, if not difficult, task to accomplish, particularly on a continuous yield basis. One of the earliest U.S. patents in the field of applying an electric field is the patent to R. C. Miller, U.S. Pat. No. 3,407,309, which issued in October, 1968.
So far, there are presently several ways to form the periodic domain pattern of desired spontaneous polarization in the nonlinear crystal, i.e., processing regions or domains having a ferroelectric polarization direction that is dominant over all other possible directions. These several ways may be classified, in part, as (1) inverted domain patterns of differing composition, i.e., by surface impurity diffusion or by ion exchange, (2) inverted domain patterns of same composition, i.e., electric field treatment with or without heat, and (3) inverted domains through periodic modulation during crystal growth, i.e., current bias or temperature fluctuation treatment during crystal growth (e.g., by a modified Czochralski process) and (4) electron beam treatment.
An example of the first type of classification is U.S. Pat. No. 5,036,220, now reexamination certificate B1 5,036,220. The first type of classification is generally achieved by the introduction into or by the removal of material from the solid body of the crystal. A most common example is titanium (Ti) diffusion through the z.sup.+ surface of the crystal. The resulting inverted domain pattern is generally only possible in a shallow surface layer and does not provide good vertical wall boundaries in the crystal.
Examples of the second type of classification is U.S. Pat. Nos. 3,407,309 and 5,193,023 and the article W. K. Burns et al., entitled "Second Harmonic Generation in Field Poled, Quasi Phase Matched, Bulk LiNbO.sub.3 ", IEEE Photonics Technology Letters, Vol. 6(2), pp. 252-254, February, 1994. The second type of classification is generally achieved by the application of a high voltage, electric field through the employment of a pattern of electrodes formed on one major surface of the crystal with a planar electrode formed on the opposite major surface of the crystal forming the opposing field electrode. The applied field is pulsed or cw for a short period of time and is generally accompanied with an applied temperature such as above 100.degree. C. The permanent inversion of the domains is accomplished by means of minute changes in ions in the unit lattice of the crystal due to the application of the electric field. By "permanent", what is meant is that the inverted domain pattern will remain as long as the crystal is not subsequently reheated to high temperature near the Curie temperature of the crystal or subjected to any further high voltage fields.
In about 1963, R. C. Miller recognized that inverted domains could be formed in ferroelectric crystals by cycling an applied electric field to switch the spontaneous polarization of the crystal to form inverted domains. U.S. Pat. No. 5,193,023 teaches periodic poling, using a pattern of electrodes on one side of a crystal and a planar electrode on the opposite side of the crystal across which an electric field is applied. In the examples of U.S. Pat. No. 5,193,023 where an electric field is employed, poling is accomplished in an atmosphere containing oxygen with an applied temperature in the range of 150.degree. C. to 1200.degree. C. and an applied voltage field of several hundreds of volts per centimeter or less.
The field inversion in U.S. Pat. No. 5,193,023 is accomplished at relatively lower applied voltages, such as at several hundreds of volts per centimeter (or several kilovolts per centimeter when using pulse voltages) or less, since the crystal is heated to a sufficiently high temperature during the applied E-field process. However, higher voltages can be successfully employed at room temperature, as demonstrated in the articles of Jonas Webjorn et al., Quasi-Phase-Matched Blue Light Generation in Bulk Lithium Niobate, Electrically Poled via Periodic Liquid Electrodes, Electronic Letters, Vol. 30(11), pp. 894-895, May 26, 1994 and "Electric Field Induced Periodic Domain Inversion in Nd.sup.3+ -Diffused LiNbO.sub.3 ", Electronic Letters, Vol. 30(25), pp. 2135-2136, Dec. 8, 1994, which are incorporated herein by reference. This second type of classification, particularly as illustrated in U.S. Pat. No. 5,193,023, has also been discounted by others in the past due to problems of electrodiffusion or due to electrode contamination and migration of electrode contaminants into the crystal during the application of an applied high volt-3 age field. However, the employment of liquid electrodes can help avoid such problems, as disclosed in the articles of Jonas Webjorn et al., supra.
Examples of the third type of classification are, respectively, the articles of A Feisst et al., "Current Induced Periodic Ferroelectric Domain Structures in LiNbO.sub.3 Applied for Efficient Nonlinear Optical Frequency Mixing", Applied Physics Letters, Vol. 47(11), pp. 1125-1127, Dec. 1, 1985 and Duan Feng et al., "Enhancement of Second Harmonic generation in LiNbO.sub.3 Crystals With Periodic Laminar Ferroelectric Domains", Applied Physics Letters, Vol. 37(1), pp. 607-609, Oct. 1, 1980.
An example for the fourth type of classification is the article of H. Ito et al., "Fabrication of Periodic Domain Grating in LiNbO.sub.3 by Electron Beam Writing for Application of Nonlinear Optical processes", Electronic Letters, Vol. 27(14), pp. 1221-1222, Jul. 4, 1991.
Of all of the foregoing classifications, the second type of classification has been found the most successful from the standpoint of providing periodic domains that have accurate periodicity and substantially vertically formed domain walls creating the nonlinear periodic waveguide in the crystal. The use of the applied electric field permits the formation of domains that have accurate periodicity and the domains are formed through the crystal forming domain walls that have some parallelism with the z axis of the crystal. However, in the case of the second type as well as all other types classified, the processing only provides for shallow domain structures that do not effectively extend through the crystal bulk and do not form vertical wall boundaries for the formed inverted domains substantially parallel with the z axis of the crystal. What is needed are high voltage processes that are room temperature applicable that provide for vertically formed domain walls that extend in the z axis direction through the crystal bulk without walkoff, i.e., capable of providing bulk frequency conversion, forming highly uniform periodicity, laterally extending domain patterns which achieve first order intervals over long crystal interaction lengths.
Therefore, it is an object of this invention to provide a nonlinear frequency waveguide converter that is fabricated by electric field poling at room temperature.
It is another object of this invention to provide for highly uniform periodicity, laterally extending domain patterns in ferroelectric crystal materials having first order interval capability over long crystal interaction lengths.
Another object of this invention is the provision of a process for high voltage field poling at room temperature of a ferroelectric crystal material useful for bulk frequency conversion.